NF3 is a gas used in the manufacture of displays, semiconductors and photovoltaics. One undesirable property of NF3 is its Global Warming Potential (GWP) which is 17,000 times greater than the GWP of CO2.
Background atmospheric abundances and trends of nitrogen trifluoride (NF3), a potent anthropogenic greenhouse gas, have been measured for the first time by Weiss, R. F., J. Mühle, P. K. Salameh, and C. M. Harth (2008), Nitrogen trifluoride in the global atmosphere, Geophys. Res. Lett., 35, L20821, doi:10.1029/2008GL035913. The mean global tropospheric concentration of NF3 has risen quasi-exponentially from about 0.02 ppt (parts-per-trillion, dry air mole fraction) at the beginning of the measured record in 1978, to a Jul. 1, 2008 value of 0.454 ppt, with a rate of increase of 0.053 ppt/yr, or about 11% per year, and an interhemispheric gradient that is consistent with these emissions occurring overwhelmingly in the Northern Hemisphere. This rate of rise corresponds to about 620 metric tons of NF3 emissions globally per year, or about 16% of the global NF3 production estimate of 4,000 metric tons/yr.
Concern about the growing concentration of NF3 in the earth's atmosphere has created a need for NF3 manufacturing methods which reduce or eliminate NF3 emissions to the atmosphere.
Manufacturing methods for NF3 often employ a temperature swing adsorption (TSA) to remove impurities which would otherwise solidify during the NF3 cryogenic distillation unit operation.
In a conventional TSA cycle, a large portion of the NF3 gas that is co-adsorbed with the impurities is vented during the regeneration step.
The following patents are representatives on refining NF3 by adsorption.
U.S. Pat. No. 4,156,598 describes the conventional TSA process used in NF3 refining: “Normally the adsorbers are operated until there is a detectable quantity of nitrous oxide leaving the adsorber at which time the adsorber is switched to the regenerated adsorber. The spent adsorber is then regenerated by permitting nitrogen to flow through the adsorber at an elevated temperature.” Using this process the co-adsorbed NF3 is vented along with the adsorbed impurities, thereby causing NF3 to be released to the atmosphere and lowering the yield of NF3.
U.S. Pat. No. 4,933,158 describes an improvement to U.S. Pat. No. 4,156,598 wherein the adsorber or the absorber is selected to try to maximize the adsorber capacity for the impurities so that the “the loss of NF3 by the adsorption is much smaller”. However, the amount of NF3 lost per cycle is roughly equivalent since the capacity of NF3 on the improved adsorber is similar. No process improvement is described to recover this co-adsorbed NF3.
U.S. Pat. No. 5,069,887 describes a process where NF3 is adsorbed and impurities (CF4) are not adsorbed. In a second step, helium is used to purge the voids of impure NF3. In a third step the pressure is reduced to desorb the NF3 which is condensed in a LIN-cooled trap. Impurities are not co-sorbed so no recovery separation is needed and column effluent is not recycled to the feed.
U.S. Pat. No. 5,417,742 describes the process recovery of perfluorocarbons (PFC) from gas streams using adsorption in TSA or Pressure Swing Adsorption (PSA) modes. The PFC in a permanent gas are adsorbed on a zeolite, then feed flow is stopped and the temperature is raised and/or the pressure lowered to desorb the PFC which is transferred to further purification or sent to another process to be used captively. During the desorption step, the effluent is not combined with the impure gas stream. Impurities are not co-sorbed on the zeolite so no recovery separation is needed.
U.S. Pat. No. 5,425,240 describes an improved TSA process for the purification of O2 in which the adsorption is performed at cryogenic temperatures. The regeneration step is conducted with product gas, i.e. pure O2, so a substantial amount of purified product gas is vented.
The following patents are representatives on pressure swing adsorption (PSA) where the column purge is recycled to the feed.
U.S. Pat. No. 5,254,154 describes an improved PSA process in which during the regeneration step, the column is depressurized countercurrent and a portion of withdrawn residual gas is mixed with the impure gas to be treated. This would be ineffective with NF3 since it is a strongly adsorbed species. The impurities are also more strongly adsorbed than NF3 and would also not be removed with a depressurization step. For the NF3 process, heating of the adsorber is a requirement during the regeneration portion of the cycle.
U.S. Pat. No. 5,620,501 describes an improved PSA process in which the void-space gas is stored in an intermediate vessel during evacuation of the adsorber (regeneration). This improvement requires that the product gas and sorbed impurity gas be easily removed by pressure reduction of the adsorber. Since both NF3 and the adsorbed impurities are strongly bound on the adsorber, heat input is required.
U.S. Pat. No. 5,254,154 describes an improved PSA process in which during the regeneration step, the column is depressurized countercurrent and a portion of withdrawn residual gas is mixed with the impure gas to be treated. This would be ineffective with NF3 since it is a strongly adsorbed species. The impurities are also more strongly adsorbed than NF3 and would also not be removed with a depressurization step. For the NF3 process, heating of the adsorber is a requirement during the regeneration portion of the cycle.
U.S. Pat. No. 5,620,501 describes an improved PSA process in which the void-space gas is stored in an intermediate vessel during evacuation of the adsorber (regeneration). This improvement requires that the product gas and sorbed impurity gas be easily removed by pressure reduction of the adsorber. Since both NF3 and the adsorbed impurities are strongly bound on the adsorber, heat input is required.
This invention is a novel TSA cycle in which 10-100% of the co-adsorbed NF3 is recovered. In the TSA cycle, a control scheme is used to stop the adsorption prior to the saturation of the adsorber with impurities and use an inert purge gas (either co-current or counter-current) to release 10-100% of the NF3 off the saturated adsorber. The effluent of the inert purge gas can be combined with the effluent of the on-stream vessel or can be recycled to the feed of the on-stream vessel.
The benefits of this novel TSA cycle is that 10%-100% of the co-adsorbed NF3 is not vented to the atmosphere but made available as product. Thus the overall process yield of NF3 is increased.
An additional benefit is in prolonging the useful life of the adsorber. When heated rapidly with adsorbers during the regeneration step in a conventional TSA, NF3 can decompose and react with the adsorber structure thereby degrading the material's performance in adsorbing impurities. The recovery step removes the NF3 in a controlled fashion reducing or preventing adsorber degradation.